Patent GB-816796-A (GB)
Title: Dehydration of isopropanol by azeotropic distillation
Filed: 1956-12-30
Abstract: <PICT:0816796/III/1> Aqueous isopropyl alcohol, particularly that produced by the hydration of propylene, is dehydrated and freed from extraneous odoriferous contaminants by azeotropic distillation using a relatively large proportion of diethyl ether as water-entrainer, the distillation being carried out at elevated pressure; at least part of the ether layer of the distillate is returned to the distilling column as reflux, and the purified isopropyl alcohol product is withdrawn from the base of the column.  Commercial grade ether may be used initially; for it undergoes purification while in use.  Crude aqueous isopropanol, such as the constant boiling mixture obtained by atmospheric distillation which contains about 91 per cent isopropanol and may contain small amounts of acetone and other impurities, is passed through heat-exchanger 2 and preheater 4, where it is heated up to about 300 DEG  F.  The crude alcohol is then introduced through line 6 into tower 7 where it is stripped of air, together with some alcohol and water, by a small portion of the vapour, supplied via line 9, from reboiler 21 of the dehydration tower 8.  The gases from the top of tower 7 are passed to a condenser; and the aqueous isopropanol condensate is returned to feed-storage, while the gas is sent to a flare stack.  The deaerated alcohol flows through line 5 into tower 8 which contains 50 plates, and has been charged with diethyl ether.  A pressure of 160-170 p.s.i.g. is maintained in the tower by means of inert gas, e.g. high-pressure oil refinery gases, applied on surge-drum 44.  The temperature at the base of tower 8 is maintained at 330 DEG  F. by means of steam-heated reboiler 21.  The ether/water azeotrope leaves the top of the tower, which is at about 260 DEG  F.; and the vapours are condensed in condenser 18.  Dry alcohol product, containing acetone, if present in the feed, is removed from the tower via line 14, and is cooled to 120 DEG  F. by passage through heatexchanger 2. The steam-condensate from reboiler 21 is passed through preheater 4, and is then used as coolant in condenser 18, whereupon low-pressure steam is evolved and taken off through line 19.  In order to obtain 1 lb. of low-pressure steam from 1 lb. of high-pressure steam, it may be necessary to by-pass a small proportion of the cold feed around preheatercooler 2.  The condensed azeotrope enters decanter 24, where an upper layer of ether is separated from a lower layer of water. The ether is pumped back to the top of tower 8 via lines 26 and 10 in order to maintain the ethercontent of the liquid on the plates in the upper half of the column greater than 70 mol. per cent.  The water, withdrawn from the decanter, is passed to stripper 30 for removal of dissolved ether by means of open steam. The ether-water overhead vapours are condensed, and the condensate collects in decanter 38. The water layer from this decanter is refluxed to the stripper, while the ether is sent to storage drum 40 or 42.  Water, which contains impurities extracted from the alcohol and the ether, is purged from the base of the stripper.  Make-up ether is intermittently supplied to decanter 24 from drum 40 or 42.  Relatively impure ether may be used when 98 per cent isopropanol is satisfactory as product.  The ether undergoes "self-purification" as it is recycled in the process, and can then be used for the production of 99.9 per cent isopropanol.